When moving the object along a given axis, the coordinates change along others axes
The coordinates you're reading are referred to the bone's local space: if your bone were exactly aligned to the world coordinates (in edit mode, during the rigging) you would have your translation matching; if, more commonly, the bone is oblique, a translation along one axis is, relative to the bone local space, a movement along more than one axis.
This is the reason why it's common, during rigging, add "extra control" bones aligned to the world coordinates, to act as parents of real control bones, making editing easier, expecially with the graph editor.